A typical nuclear reactor fuel assembly includes top and bottom support members having a multiplicity of fuel rods and control rod guide tubes supported therebetween. Each fuel rod and control rod guide tube is separately held against lateral displacement by grids, generally of an egg crate configuration, which are axially spaced along the fuel assembly length. Since the fuel rods and control guide tubes are usually made of Zircoloy and the grid assemblies used for supporting these components are usually made of Inconel, the incompatibility of the materials requires that the grids be held in position along the fuel assembly length by mechanical means, rather than brazing, welding or the like.
In one well-known grid design short sleeves, which correspond to the number of control rod guide tubes in the fuel assembly, are brazed at appropriate points in grid assembly cells which are formed by interleaved grid straps. Each sleeve projects on the order of about two inches beyond the edge of a grid strap. During assembly of the fuel assembly, the grids are mounted in an axial predetermined position and after the control rod guide tubes are pulled through the grid sleeves, a bulging tool is moved into the control rod guide tube and stopped at a point just below a grid strap, but still inside the sleeve which extends through the grid cell. The tool is then expanded to cause projections on the tool to plastically deform the control rod guide tube and sleeve. The bulging tool is then moved to a point just above the grid, and the process of plastically deforming the material again repeated with the result being that the grid is mechanically locked and rigidly secured to the control rod guide tubes in the fuel assembly.
Prior art bulge tools, such as that described in U.S. Pat. No. 4,182,152, comprise a cylindrical housing having axially extending tines formed by slots cut into the walls of the cylindrical housing. Projections are integrally formed in the outer surface of the tines near the end of the cylindrical housing. These projections are made to move radially outward under the influence of an internally operating ram to form bulges in a sleeve and guide tube while plastically deforming the material thereof. As the sleeve and guide tube material is deformed by the action of the ram riding on the complementary inner surfaces of the tines, the inner diameter of the tubes is held to a predetermined minimum by a coacting effect of other tines, located between the tines having projections, and the ram surfaces.
In the above-described prior art fuel assemblies, the sleeve and guide tubes are deformed with dimple-like bulges circumfunctially spaced about 90.degree. apart to capture the guide tube with respect to the grid straps. The guide tubes are therefore subject to local stresses at the bulges. In addition, the prior art expansion tools require several operations to create all of the necessary bulges to securely capture the guide tube at the various axially spaced grid locations in the fuel assembly.